Devices for the delivery of medicament in an automatic way e.g. auto-injectors, are known as convenient and safe devices for patients to self-administer various drugs themselves. For safety reasons, primarily to guard against needle sticks, many devices for the delivery of a medicament include covers and other components that protect users before and after use of the injector. Although different devices for the delivery of medicament vary in their total feature sets, they all have a mechanism that delivers the contents of a preloaded, prefilled container automatically, i.e., without requiring a person to manually force the contents within a container through a delivery member, e.g., a needle or a nozzle, into the patient.
Auto-injectors are described in U.S. Pat. No. 5,478,316 to Bitdinger et al.; U.S. Pat. No. 7,112,187 to Karlsson; and U.S. Pat. No. 7,125,395 to Hommann et al.; U.S. Patent Application Publication No. 2007/0021720 to Guillermo; and International Publication No. WO 2006/057604 by Olson, for example. The automatic delivery mechanism in a device for the delivery of medicament usually includes a compressed helical compression spring that drives a plunger rod forward as the spring decompresses upon activation of the device. In some designs, such springs work well. A fully compressed helical spring provides a force that is large enough to overcome the static friction between the plunger and the inner wall of a container, so called break loose force, and the spring at its full extension, which is often not at its full decompression, provides a force that is large enough to complete the injection stroke but causing the probability of the container breakage.
A challenge in the design of such springs is balancing the need for sufficient force at the end of stroke against the need for a not-too-large force during storage (when the spring is fully compressed) that may overload the other components in the device. Such other components may be made of plastics, glass or similar materials that have limited strengths.
Thus, the drive mechanism in such a device for the delivery of medicament should meet the following simultaneous goals: 1) exert enough force to overcome the “break loose” force of the container stopper and initiate delivery of the medication, 2) exert enough force to complete—the injection stroke, 3) meet goals 1) and 2) in an acceptable time frame (usually a few seconds), and 4) exert a low force during storage.
Rather than use typical helical compression springs, some delivery devices use constant force springs or even variable force springs in an attempt to meet the goals mentioned above. For example, in our earlier issued U.S. Pat. No. 8,460,245, such variable force tension springs are discussed.
Although constant-force or variable springs are better able than compression helical springs to meet the design goals of injector devices, there is also a need to provide a smooth and reliable way to trigger the device to initiate the injection process. Visually indicating the start of the injection process and showing the progress of the injection are also important in the design of a medicament delivery device, such as an auto-injector. Likewise, after the injection is complete it is important for safety reasons to protect the user and others by preventing accidental needle sticks. Preferably, the covering of the used needle should be automatic and nonreversible. In other words, the design should not require the user to perform any manual manipulation of any component of the device, especially components located at the distal or needle end of the device. Our invention addresses these concerns while employing a variable force spring as the driving force to perform the injection.